Powering spaceprobes

The heat is on

A more efficient way to make electricity in spacecraft

LARGE solar panels are often the most noticeable bits of a spacecraft, just as the many sails of a 19th-century tea clipper lent that craft its distinctive character. But once merchant ships could drop their sails in favour of steam engines, they did so. The wind, after all, is not that good an energy source. Nor is the sun. Though it shines constantly, it dims as the square of your distance from it, so deep-space probes need a different source of power.

At the moment, this is provided by a process called thermoelectric coupling. But Scott Backhaus, of the Los Alamos National Laboratory, in New Mexico, and his colleagues, propose to change that by reviving an orphan invention from the age of sail—the Stirling engine.

In 1816, a British clergyman called Robert Stirling proposed a competitor to the then-ubiquitous steam engine. It used heated air instead of steam as the “working fluid” that drives the piston and—crucially for Dr Backhaus's needs—it was a sealed system, in which the air was endlessly recycled.

Thermoelectric coupling uses heat from the decay of a radioactive fuel (as opposed to the full-scale nuclear fission which powers a reactor on Earth) as its energy source. It converts this directly into electricity. But the process is only 7% efficient. The Stirling-based “thermoacoustic” system which Dr Backhaus is developing, and which is described in the latest edition of Applied Physics Letters, has the same heat source, but converts 18% of it into electricity.

The main difference between the thermoacoustic system and Stirling's original idea is that, instead of using thermal expansion of the working fluid to push the piston, it uses powerful pulses of sound. A minor difference is that the working fluid is helium, rather than air. The helium is heated by sending it through a carefully designed “regenerator”—a stack of 322 discs of stainless-steel wire-mesh connected to the heat source. The design of the regenerator causes the helium to expand and contract in a way that produces a powerful sound wave. This drives the piston back and forth so that it acts as an alternator, generating electricity.

Like fuel cells, another 19th-century invention that promised more than it delivered, the Stirling engine has been a device in search of an application. And, also like fuel cells, that initial application may now be found in the specialised needs of space travel. Fuel cells have gone on to great things. Perhaps Robert Stirling's time has come at last.